As a side note, it's not unheard of for lake bottoms to trap gasses and such that bubble up from below the ground. Rather than bubble its way up to the surface, the gas kept at the bottom through the sheer downward pressure exerted by the water.

Sadly, when that environment is somehow disturbed, the trapped gasses have been known to rush up to the surface in an enormous, invisible cloud of death. Any nearby inhabitants unlucky enough to be at ground level are killed instantly.

The effect is similar to the "choke damp" or "blackdamp" phenomenon experienced by miners, who unknowingly walked into these invisible gas clouds only to die instantly.